The subject matter disclosed herein relates to shrouded axial fans. More specifically, the subject matter disclosed herein relates to structure to enhance stall margin of shrouded axial fans.
Axial flow fans are susceptible to leakage flow from the high pressure side to low pressure side of the fan blades, typically a flow from the downstream side of the fan to the upstream side of the fan. The leakage flow occurs at either the fan blade tip, specifically between the tip and the casing in an unshrouded fan, or between the shroud and the casing in the case of a shrouded fan. This leakage flow is reingested into the fan at, for example, a front clearance gap between the shroud and the casing, at a leading edge of the shroud. As the leakage flow reenters the fan, it gives rise to rotating swirl flow and instabilities at the blade tip, often causing the flow at the blade tip to separate and stall prematurely. The result is a generally limited stable operating range for a typical axial flow fan that is limited in its range of applications. Many configurations of “casing treatments” have been developed to address the leakage flow issue, most of which are specifically applicable to unshrouded axial fans or impellers used in high-speed compressor applications, while only a limited number are suitable for use with shrouded fans. In one such case, a number of vanes extend from the interior of the casing toward the shroud to reduce swirl in the recirculating flow.